(a) Field of the Invention
This invention relates to apparatus for preventing the loss of drilling mud when the kelly is disconnected from the drill pipe.
(B) Description of the Prior Art
In the drilling of oil and gas wells, it is common practice to insert in the drilling string between the kelly and the drill pipe a valve to retain mud in the kelly when the drill string is broken. The advantages of using such a valve are well known and include saved mud cost, decreased chances of pollution, and increased safety to rig personnel.
Typical valves of the mud retaining type are illustrated in the following patents:
______________________________________ Patentee U.S. Pat. No. ______________________________________ Taylor 3,331,385 Garrett 3,698,411 Litchfield, et al 3,738,436 Williamson 3,965,980 Liljestrand 3,967,679 ______________________________________
All of the above listed patents include a downwardly opening spring loaded poppet type valve enclosed in a body having at least two parts. These two extra pieces in the drill string replace a single piece kelly saver sub, which functions to reduce wear on the kelly pin. The two-part body is generally longer than a standard kelly saver sub and consequently increases the length of the string which must be handled at the rig. In most offshore operating areas, it is mandatory that a lower manually operated kelly safety valve be included in the string at all times, which is another addition to the length of the string which must be handled. Thus, on offshore rigs, where the height of the derrick or mast is usually limited, it may be impossible to include mud retaining type valve with a two-part body.
An additional disadvantage inherent in mud retaining valves with two-part bodies is that the pin of the lower body member replaces the pin of the kelly saver sub and is therefore subject to tremendous wear. This wear limits the longevity of the pin and therefore the longevity of the valve. A solution to this problem has been to insert an additional short sub below the lower body member. However, this solution is not entirely satisfactory because it adds still more length to the string.
It sometimes becomes necessary to run wire line tools into the drill string to perform various downhole operations. It is therefore necessary that the mud retaining valve have means by which wire line tools may be run therethrough. In the device of certain of the prior art, these means take the form of a threaded plug screwed into the central portion of the movable poppet. To remove the plug of the apparatus, a tool is run into the string to engage a bolt headed portion of the plug and rotated to thereby unscrew the plug. In the valves of the other above cited patents, the central portion of the movable poppet includes a cap of a frangible material that may be broken out with a sinker bar.
Neither of these means for passing tools through a mud retaining valve is entirely satisfactory. The "threaded plug" device requires a special tool for engaging and unscrewing the plug. The frangible cap of the other patents is not entirely satisfactory in that occasionally portions of the cap remain unbroken leaving jagged projections which may damage or sever the wire line. Also, the broken out portions of the cap form debris which impedes drilling.
A further disadvantage of heretofore existing mud retaining valves is in the fact that none of them include means for adjusting the force with which their respective closure members are driven upwardly. The force may be insufficient to close the valve when heavy muds are used. When lighter muds are used, the force may be so excessive as to strain the mud pumps.